The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2001-201714 filed on Jul. 3, 2001 the entire contents thereof is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a rear suspension structure for a motorcycle which is suitable for enhancing the rigidity of a swing arm while restraining an increase in the weight of the vehicle body.
2. Description of Background Art
As a rear suspension structure for a motorcycle, there is generally a structure in which a swing arm extends from the vehicle body side towards the rear side of the vehicle body. End portions of a rear shock absorber unit are fitted to the swing arm and to the vehicle body side. This structure will be described referring to FIG. 10.
FIG. 10 is a side view of a major part for illustrating a conventional rear suspension structure. A structure is provided in which respective rear portions of left and right main frames 301, 301 (the main frame 301 on the deep side is not shown) constituting a vehicle body frame 300 are connected to each other by an upper portion cross pipe 302 and a lower portion cross pipe 303. The upper portion cross pipe 302 is provided with a bracket portion 304 projecting rearwardly. An upper end portion of a rear shock absorber unit 306 is fitted to the bracket portion 304. A lower end portion of the rear shock absorber unit 306 is fitted to a lower portion of a swing arm 308 through a link 307. The link 307 is fitted to a lower portion of the lower portion cross pipe 303 through a link 311.
The swing arm 308 is a member provided with a shock absorber inserting hole 313 for passing the rear shock absorber unit 306 therethrough.
In the above technology, the swing arm 308 is provided with the shock absorber inserting hole 313 for the rear shock absorber unit 306, so that the flexural rigidity and torsional rigidity of the swing arm 308 are reduced due to the presence of the shock absorber inserting hole 313, which is considered to affect drivability and stability at the time of cornering, for example.
However, the measure of increasing the size of the swing arm 308 or merely fitting a reinforcement member to the swing arm 308 would increase the weight of the vehicle body or impair the motion performance of the vehicle.
Accordingly, it is an object of the present invention to enhance the rigidity of the swing arm while restraining an increase in the weight of the vehicle body, by improving the rear suspension structure for motorcycle.
In order to attain the above object, according to the present invention a rear suspension structure includes a swing arm having one end swingably fitted to a pivot shaft provided on the vehicle body side. A wheel is fitted to the other end of the swing arm. A shock absorber unit includes an upper end fitted to the swing arm with the lower end of the shock absorber unit connected to the vehicle body side. Arm portions extending in the front-rear direction are provided on the left and right sides of the swing arm. A window portion for passing the shock absorber unit therethrough is provided between the arm portions, a cross beam is provided between the arm portions as a bridge. The upper end of the shock absorber unit is fitted to the cross beam and the whole part or a part of the window portion is closed with the cross beam.
Since the whole part or a part of the window portion of the swing arm is closed with the cross beam, the swing arm can be reinforced by the cross beam, and the flexural rigidity and torsional rigidity of the swing arm can be enhanced. Moreover, since the upper end of the shock absorber unit is fitted to the swing arm side, it is unnecessary to provide, for example, a fitting portion for the upper end of the shock absorber unit on the vehicle body frame side. Therefore, an increase in weight can be restrained.
According to the present invention, the cross beam is trapezoid shaped in front view, and the upper end of the shock absorber unit is fitted to the upper side of the cross beam.
Since the cross beam is trapezoid shaped in front view, vertical forces exerted on the cross beam upon extension or contraction of the shock absorber unit can be received roughly as a tensile force or a compressive force by slant sides of the trapezoid-shaped cross beam.
For example, where the cross beam is included as a straight member and the spacing between left and right arm portions of the swing arm is wide, the cross beam becomes longer, and a greater flexural moment is generated in the cross beam. In contrast, according to the present invention, the flexural moment can be decreased, and the rigidity of the cross beam against the extension and contraction of the shock absorber unit can be enhanced.
The fitting of the upper end of the shock absorber unit to the swing arm and the fitting of the lower end of a rear shock absorber unit to the vehicle body side are performed respectively through spherical sliding bearings.
By fitting the upper end and the lower end of the shock absorber unit through the spherical sliding bearings, inclinations of the shock absorber unit with reference to the swing arm side and the vehicle body side can be absorbed, so that excessive external forces can be prevented from acting on the shock absorber unit itself, on the swing arm side or on the vehicle body side.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.